The French government has declared an "ecological disaster" in one of Europe's most unique and beautiful nature reserves after an oil spill from an underground pipeline spread over acres of pristine habitat.

More than 4,000 cubic metres of crude burst into the Coussouls de Crau in southern France on Friday, prompting an emergency clean-up by firefighters to try to save its thousands of birds and dozens of rare species.

Chantal Jouanno, a junior environment minister, said the spill posed an unquestionable threat to the natural life of the reserve. "This is a real ecological disaster because we are on a protected site," she told journalists after a visit to the area.

Targeting the pipeline's operators, the South European Pipeline Company (SPSE), Jouanno said an investigation had been opened to establish the accident's cause. "Obviously the operating company is responsible, and we will do all that is necessary to see if they've been negligent," she said.

Situated on the edge of the Camargue national park, the Coussouls de Crau has a reputation for being as important a refuge for the rare birds of Europe and northern Africa as its celebrated neighbour is for the region's wild bulls and horses.

Its 18,278 acres (7,400 hectares) of plains are described by local scientists as the last remaining dry steppe in Europe and, while the five-acre spill is not thought to have had any effect on the water supply of residents, experts fear that it could be lethal for the delicate flora and fauna.

Laurent Tatin, the scientific director of the reserve, said it could prove especially destructive for species which depend upon the particular habitat, such as the pin-tailed sandgrouse, whose only French home is the reserve, and the protected Crau grasshopper, which is found exclusively in the area.

Other rare species reliant on the reserve include the ocellated lizard, the lesser kestrel and the calandra lark, which is being kept from the brink of extinction in France by its small but steady population in the Coussouls.

Yesterday firemen and representatives of SPSE were desperately trying to carry out a clean-up operation aimed at ridding the reserve of the toxic leak as quickly as possible. But even as the pipeline operators were defending their role in the spill, they faced accusations from one of the reserve's wardens that their response to the disaster had been too slow.

Guillaume Paulus, a reserve warden who was the first person to witness the eruption from the pipeline at about 7.30am on Friday, told journalists that the pipeline was "spitting out oil about three or four metres in the air" and that he was surprised no one from the operating company had noticed the leak before he himself called to alert them.

A spokeswoman for SPSE said it was too early to determine what triggered the breakage, insisting that regular maintenance had been carried out since the pipeline was built in 1971 and that no sign of damage had been recorded after the last security check.

A separate spokesman, François Trabucco, told journalists that as there was no river in the area it was "impossible to tell you here and now that it's a cause of concern for the environment".

SPSE, whose shareholders include the oil giants BP, ExxonMobil and Total, built the pipeline as a means of linking the French town of Fos-sur-Mer with Karlsruhe in Germany. Its website says that it supplies 23m metric tonnes of crude oil to refineries and petrochemical plants in France, Germany and Switzerland every year.